The present invention relates generally to speed sensing devices for rotating equipment and, in particular, to a speed sensing device that also detects rotational direction of the rotating equipment.
Vehicle wheel speed sensor systems are well known. Rotational speed is used for numerous measurement devices and control systems, including vehicle speedometer readings, vehicle cruise control feedback, and vehicle antilock braking system feedback. Wheel speed sensors can be mounted in any of several locations, such as in wheel bearing assemblies, and in the differential.
Speed sensor systems typically operate by means of a target mounted to rotated with the rotating equipment. This target is paired with, and is separated from, a stationary sensor by an air gap. The stationary sensor generates a signal when the target rotates past it. There are indices around the periphery of the target. The number of times the indices pass by the sensor, or the frequency of signals generated, in a given period of time, is then converted to a rotating speed and passed on to the appropriate measurement device or control system.
Vehicle wheel speed sensing systems are typically grouped into two types, active sensor systems and passive sensor systems. Passive sensors do not require a power supply in order to operate. In a passive sensor system, the stationary sensor is a permanent magnet that projects a magnetic field into the air gap. The stationary sensor detects a change in the magnetic field""s reluctance caused by the moving target, typically a toothed wheel made of ferrous material, as it passes through the magnetic field. The output of the passive sensor is a raw analog signal that varies greatly with the rotational speed of the vehicle wheel. The output of the passive sensor is also susceptible to false signals when the wheel is subjected to vibration. In addition, passive sensors are limited to a close clearance air gap of about 1 to 2 mm.
Active sensor systems represent the next generation technology to be utilized in vehicle wheel speed sensing devices. Active sensor systems typically utilize one of two technology devices, which are well known in the art as Hall Effect devices or Magneto-Resistive devices. The two technologies have been found to be comparable in terms of performance. Active sensor systems require a power supply to operate and are further divided into two categories, back-biased sensors and non back-biased sensors. Back-biased sensors generate the magnetic field from the stationary sensor, while the moving target, or encoder, is constructed of ferrous material, as in the passive sensor system. Non-back biased sensors, conversely, generate the magnetic field from the encoder. Because the magnetic field is generated from the moving target, non back-biased stationary sensors do not need magnets, require correspondingly fewer components and are thus smaller than back-biased stationary sensors. Sensors installed in either back-biased or non back-biased form detect the frequency of the changes in voltage of the magnetic field, and direct the output to the appropriate measurement device or control system. The output of the active sensor, in either back-biased or non back-biased form, is a high quality digital signal that varies between fixed values and is not affected by the rotational speed of the wheel. Active sensor control systems are smaller than passive sensor control systems, can function at zero and near zero rotational speed, are immune to false signals due to vibration, and are capable of having a greater air gap than passive stationary sensors.
A drawback to the above mentioned systems is that while they can measure rotational velocity, they do not measure the direction of rotation. But, for rotational detection relating to vehicle wheels, direction detection is becoming important in addition to the common measurement of wheel rotational velocity. This need is arising due to improvements in existing as well as new features being proposed for vehicles, including anti-lock braking, traction control, vehicle stability control, electric park brake, electro-hydraulic braking, adaptive cruise control, integrated chassis control, and in-vehicle navigation. Furthermore, due to market pressures to reduce the cost of vehicles, such a system capable of detecting both rotational velocity and direction need to perform the functions required, in harsh environmental conditions, accurately and reliably, and yet for minimal additional cost.
In its embodiments, the present invention contemplates a rotational sensor system for detecting rotational velocity and rotational direction of a rotatable member rotatable in a forward and in a reverse rotational direction about an axis of rotation. The rotational sensor system includes a rotatable element adapted to operatively engage and rotate with the rotatable member about the axis of rotation, with the rotatable element having a generally circular periphery that includes means, spaced about the periphery, for creating magnetic field variations, first sensor means, adjacent to the periphery, for sensing variations in a magnetic field and producing an electric signal in response thereto, and second sensor means, adjacent to the periphery and circumferentially spaced about the periphery from said first sensor means, for sensing variations in a magnetic field and producing an electric signal in response thereto. The system also includes means for generating a magnetic field, means for receiving the electric signals from the first sensor means and second sensor means and detecting a phase shift between the signals from the first sensor means and the second sensor means, and means for generating an output signal indicative of rotational velocity and rotational direction of the rotatable element.
The present invention further contemplates a rotational sensor system for detecting rotational velocity and rotational direction of a rotatable member rotatable in a forward and in a reverse rotational direction about an axis of rotation. The rotational sensor system includes a rotatable element adapted to operatively engage and rotate with the rotatable member about the axis of rotation, with the rotatable element having a generally circular periphery that includes means, spaced about the periphery, for creating magnetic field variations, first sensor means, adjacent to the periphery, for sensing variations in a magnetic field and producing an electric signal in response thereto, and second sensor means, adjacent to the periphery and circumferentially spaced about the periphery from said first sensor means, for sensing variations in a magnetic field and producing an electric signal in response thereto. The system also includes means for generating a magnetic field, means for receiving the electric signals from the first sensor means and second sensor means and detecting a phase shift between the signals from the first sensor means and the second sensor means, and means for generating an output signal indicative of rotational velocity and rotational direction of the rotatable element. The system further includes means for generating an output signal including a series of square waves having frequencies corresponding to the rotational velocity of the rotatable element and having amplitudes that are in a first range when the rotatable element is rotating in the forward direction and in a second range when the rotatable element is rotating in the reverse direction.
Accordingly, an object of the present invention is to provide a relatively simple rotation sensing system that can detect rotational velocity, including near zero and zero rotational velocity, as well as rotational direction.
Another object of the present invention is to provide a rotational velocity and direction sensing system with a simple and reliable signal for its output protocol.
An advantage of the present invention is that more information about a rotating member is reliably provided by the sensing system while minimizing the complexity of the system.
Another advantage of the present invention is that it is relatively simple, meets functional requirements for measuring rotational velocity, even down to zero velocity, as well as rotational direction, while operating fast and reliably in harsh vehicle environments.